Transmission lines are used to transport gas, oil, and other fluids, such as carbon dioxide, over long distances to customers or processing facilities. Large diameter transmission lines currently are made entirely of steel. The American Petroleum Institute (API) provides design guidelines for building these transmission lines, and the transmission lines also are regulated by the Department of Transportation's Office of Pipeline Safety (DOT/OPS). The API specifications for transmission lines (API 5L) provide design rules, specifications for acceptable grades of steel, and specifications for acceptable steel pipe joint construction. The API specified grades of steel for large diameter seam welded pipes are the X-series, where X-60 refers to a steel grade with a minimum yield strength of 60,000 psi (60 ksi).
Transmission lines--particularly those used to transport natural gas--are required to withstand higher and higher pressures and to transport more and more gas. The increased demands have led to the use of higher strength, lower toughness steels, such as X-80 steel, to manufacture the transmission lines. Unfortunately, the use of steels with lower toughness increases the potential for ductile rupture of the pipeline.
The danger of ductile rupture can be reduced somewhat by wrapping the steel transmission line with fiberglass reinforced plastic to prevent the propagation of ductile rupture-type fractures. However, fiberglass reinforcement by itself provides only protection against crack propagation along the pipeline. In order to provide more cost effective protection against ductile rupture, methods are needed which will (a) allow for a reduction in the thickness of the steel liner wall, and (b) provide for load sharing between the steel liner and the fiberglass reinforcement.
A technique called "autofrettage," which is practiced in the manufacture of composite wrapped pressure vessels, theoretically can be used to advantage. During autofrettage, a fiberglass composite wrapped metal vessel is subjected to an internal pressure greater than the pressure at which the metallic liner experiences plastic deformation. Once plastic deformation or yielding of the metallic liner occurs, the pressure is reduced. The resulting composite wrapped vessel is left with a relatively consistent residual stress pattern in which the metallic liner is in circumferential compression and the fiberglass composite wrap is in tension. This residual stress pattern (a) allows the fiberglass composite material, which has a much lower stiffness than the metallic liner, to carry a substantial portion of the pressure load, and (b) reduces the circumferential tensile stress on the metallic liner at the operating pressure of the vessel.
Autofrettage cannot be easily applied to transmission lines. In pressure vessels, the yield strength of the metallic liner is tightly controlled, and the use of a single autofrettage or "proof" pressure results in a consistent residual stress pattern within each vessel. In contrast, the only tightly controlled property for steels used to make transmission lines is the "minimum yield strength." In the art of manufacturing fluid transmission lines, "Specified Minimum Yield Strength" (SMYS) means that no portion of the pipeline can have a yield strength below the specified strength. The pipeline commonly has portions with a yield strength above the specified strength. For example, a pipeline made of X-60 steel may have portions ranging in yield strength FROM about 60 ksi up to about 75 ksi, with some welded areas having a yield strength as high as about 80 ksi.
Because of the potential variability in yield strength along a transmission line, the use of a single autofrettage or "proof" pressure does not necessarily result in a consistent residual stress pattern. Because of this, autofrettage has not been considered a viable method to reduce the danger of ductile fracture in transmission lines.
Methods are needed for (1) minimizing the thickness required of the metallic liner while maximizing the resistance to ductile fracture propagation, and (2) producing a consistent residual stress pattern effective to resist ductile fracture propagation along the longitudinal axis of a length of pipeline having variable yield strengths.